In a method which is known in the art, uninterrupted combustion of the waste materials takes place on a grate, and in addition to considerable quantities of dust a whole range of substances contained in the waste are released in gaseous form, as aerosols or as dusts with the resulting flue gas and have to be separated out or washed out of the flue gas with enormous outlay after utilisation of the heat by way of waste heat boilers. In this case for instance the following purification steps are carried out after the waste heat boiler: dust removal, two-stage scrubbing with evaporation of the washing water above all for the gaseous harmful gases, adsorption stage for dedioxination, denitrogenation apparatus with heat exchanger.
Adherence to the legally prescribed limiting values necessitates such high outlay on the process and thus overall necessitates a very cost-intensive treatment of the residual waste,
A method of heat treatment of waste materials is also known from EP-B-0 111 081, in which
a) the waste materials are subjected to pyrolysis in a first heat treatment zone, PA0 b) the pyrolysis gas produced thereby has dust removed to a certain degree in a dust-collecting cyclone, PA0 c) the dedusted pyrolysis gas is burnt in a second heat treatment zone, PA0 d) the flue gas produced in the second heat treatment zone is cooled and energy is recovered, and PA0 e) then the flue gas is purified of harmful substances in a filter by the addition of basic material. PA0 a) the waste materials are subjected to pyrolysis in a first heat treatment zone, PA0 b) at least 90%, preferably at least 95% of the dust is removed from the pyrolysis gas produced thereby, PA0 c) the dedusted pyrolysis gas is burnt in a second heat treatment zone, PA0 d) the flue gas produced in the second heat treatment zone is cooled, PA0 e) then the cooled flue gas is denitrogenated catalytically, and PA0 f) finally is purified of harmful substances in a filter by the addition of appropriately reactive additives. PA0 a) the waste materials are subjected to pyrolysis in a first heat treatment zone, PA0 b) at least 90%, preferably at least 95% of the dust is removed from the pyrolysis gas produced thereby, PA0 c) the dedusted pyrolysis gas is burnt in a second heat treatment zone, PA0 d) then the flue gas produced in the second heat treatment zone is denitrogenated non-catalytically, PA0 e) the denitrogenated flue gas is cooled and PA0 f) finally is purified of harmful substances in a filter by the addition of appropriately reactive additives. PA0 a) the waste materials are subjected to pyrolysis in a first heat treatment zone, PA0 b) at least 90%, preferably at least 95% of the dust is removed from the pyrolysis gas produced thereby, PA0 c) the dedusted pyrolysis gas is burnt in a second heat treatment zone, PA0 d) the flue gas is cooled and PA0 e) finally is purified of harmful substances in a filter by the addition of appropriately reactive additives.
However, the flue gas finally discharged to the exterior via a chimney using this method already fails to meet the legal requirements, particularly with regard to the content of NO.sub.X, gaseous harmful gases and heavy metals, for example mercury, and these requirements will be tightened further in the next few years.
It has already been proposed to provide a denitrogenation apparatus at the end of the method chain for reducing the NO.sub.X content in the flue gas.
In processes for heat treatment of waste the catalytic denitrogenation is carried out as a rule in the optimum temperature range of approximately 300.degree. C. However, at the end of the method chain the temperature of the flue gas is usually approximately 70 to 130.degree. C. This means that the denitrogenation apparatus currently in use has to be additionally provided with a heat exchanger with a natural gas heating arrangement in order first of all to heat up the flue gas to the temperature necessary for the denitrogenation and then to cool it again before it is discharged through the chimney.
The arrangement of the denitrogenation apparatus in a part of the method chain which no longer necessitates heating of the flue gases is not possible in the methods of heat treatment of waste materials which are currently known, because during the combustion process trace elements present in the waste, such as arsenic, selenium and the like, are released and would render the denitrogenation plant incapable of functioning after a very short time.
Furthermore, in conventional heat treatment of waste materials sulphur dioxide is produced on a large scale and some of it reacts with the excess atmospheric oxygen to form sulphur trioxide. This compound in turn reacts with the added ammonia or ammonia water or urea which is necessary for the denitrogenation reaction and forms corresponding salts, predominantly ammonium hydrogen sulphate. These salts have strongly adhesive properties and as a result lead to unwanted deposits in the region of the catalyst or the subsequent heat exchangers, pipes, waste heat boilers and other units.
Consequently a SCR denitrogenation apparatus cannot be used immediately after the waste heat boiler--before the flue gas scrubbing units--in conventional plant. It can therefore only be provided at the end of the method chain, when the sulphur dioxide or trioxide content is correspondingly lowered by the flue gas scrubbing.
A further method of denitrogenation, but one which is applied much more rarely in waste technology, is the so-called SNCR method, which is carried out non-catalytically at temperatures between 850 and 1000.degree. C.
When the SNCR method is used in conventional heat treatment apparatus, denitrogenation must be carried out in the waste heat boiler because the effective "temperature window" for this denitrogenation reaction lies in the range from 850 to 1000.degree. C. However, in the region of the boiler the residence time of the flue gas in the "temperature window", which is important for the efficiency of the denitrogenation reaction, is very short, so that the necessary denitrogenation rate must be achieved by super-stoichiometric addition of ammonia. This in turn leads to the resulting products from the flue gas scrubbing having a smell of ammonia which is unwanted for all disposal routes and these products consequently have to undergo costly further treatment.
Also the particularly economical SNCR method cannot be carried out for this reason and for the reasons already mentioned of the formation of ammonium sulphide salt and also the formation of unwanted deposits due to the reaction of the added ammonia in the waste heat boiler with products contained in the dust.
The object of the invention, therefore, is to provide a method and apparatus for heat treatment of waste materials, in which the outlay on apparatus for purification of the flue gas under the prescribed limiting values is substantially reduced by comparison with the known prior art.